This invention relates to conductivity measurement, and particularly to a process of measuring conductivity of a fluid using a pH analyzer.
Conductivity measuring analyzers are well known in the art and are used to measure the conductivity of a fluid, such as a liquid or dispersion of solids in a liquid. Conductivity analyzers are used to investigate the properties of electrolyte solutions, such as the degree of disassociation, the formation of chemical complexes and hydrolysis. A contact conductivity sensor employs a pair of electrodes in contact with the electrolyte solution. The measurement circuit supplies a voltage to the electrodes that such that the resulting current between the electrodes is used to measure conductivity. More specifically, the sensor provides an output current that is a measurement of the conductivity of the solution, which is the inverse of resistivity.
In many industrial process control systems, it is necessary to measure both the conductivity of a solution, as well as its pH. Ordinarily, an ion-sensitive sensor, such as a pH sensor, and a separate conductivity sensor, are employed in the process control system. In some cases, however, a conductivity sensor is simply not available. For example, certain maintenance and diagnostic operations may require conductivity measurements that might not be needed in normal operation of the process control system. In other cases, the ion-sensitive sensor and conductivity sensor might not share the same measurement circuit, and may require separate electrical control loops, which can be expensive and difficult to retrofit into existing systems. The present invention is directed to the problem of measuring conductivity of a fluid in the absence of a conductivity sensor, and particularly to a method of operating an ion-sensitive sensor to measure conductivity of a fluid, so that the analyzer may be operated as both a pH analyzer and a conductivity analyzer.
According to the present invention, an analyzer is operated to measure conductivity of a solution. A current, IREF, is applied to the reference electrode of an ion-sensitive sensor having an ion-specific electrode. The current is a substantially constant current having first and second opposite states of substantially equal value. A peak-to-peak voltage, VION, between the ion-specific electrode and a common electrode is measured, and the conductivity of the solution, CSOL, is identified based on       C    SOL    =                    I        REF                    V        ION              .  
According to another aspect of the present invention, the analyzer operates the ion-sensitive sensor to measure pH of the sample solution and operates the ion-sensitive sensor to measure conductivity of the solution. More particularly, an average voltage, VIONxe2x88x92AVE, is measured between the ion-specific electrode and the common electrode and an average voltage, VREFxe2x88x92AVE, is measured between the reference electrode and the common electrode. The pH of the solution is identified based on a difference between the measured average voltages VIONxe2x88x92AVE and VREFxe2x88x92AVE.
According to different embodiments of this aspect of the invention, pH measurement may be accomplished simultaneously with conductivity measurement, or during mutually exclusive cycles.